How Shallow and Deep Defects Drive Carrier Dynamics in Tin-Iodide Perovskites

Abstract

Tin-halide perovskites (THP) exhibit complex carrier dynamics due to the interplay between electronic doping and carrier trapping, both of which affect device performance. Evaluating the impact of trap states is challenging because the timescales of photogenerated electron recombination with dopant holes and trapping often overlap. Here, Transient Absorption Spectroscopy (TAS) is used across a broad spectral and temporal range, spanning from visible to near-infrared and from femtoseconds to microseconds, to probe both sub-bandgap and band-edge transitions, while manipulating defect and doping densities via chemical treatments. Focusing on tin triiodide perovskites, the rapid carrier recombination due to high electronic doping density is considered the main source of carrier loss. However, deep electron trap states originated by two distinct type of defects are identified: surface Sn(IV) defects and tin interstitials. Surface Sn(IV) defects play a key role in the loss of photo-generated carriers, but their density can be mitigated by the addition of SnF₂, improving carrier lifetimes. Nevertheless, excessive SnF₂ promotes the stabilization of tin interstitial traps, highlighting a delicate balance in defect control. Moreover, near-infrared TAS reveals sub-bandgap transitions associated with shallow traps, which contribute to band-edge repopulation within tens of picoseconds. This work disentangles the contributions of doping and trap-mediated processes to the optoelectronic mechanisms in THP, offering insights into defect management for performance optimization.